This invention relates to a straightness meter utilizing holography. In a machine tool, for example, a tool slide is adapted to move in a striaght line along a guide. The degree of straightness of this movement constitutes an important factor for determining the degree of machining accuracy. In such machine tools, therefore, the tool slides must be tested for lateral displacement (namely displacement in the horizontal direction and in the vertical direction in the plane perpendicular to the direction of the tool movement; hereinafter referred to as "lateral displacement") at the time that the machine tools are assembled or subjected to test for accuracy. There have been made attempts at improving machining accuracy of machine tools by constantly monitoring the lateral displacement of their tool slides thereby controlling the straightness of their tools' movement at all times during the operation of the machine tools.
In such attempts, use is made of straightness meters. Heretofore, autocollimators and laser straightness meters (otherwise known as tooling lasers) have been utilized as optical straightness meters. For lack of accuracy or for some other fault, such conventional straightness meters have fallen short of satisfying the purpose mentioned above.
Recently, the holographic technique has been applied to the measurement of interference and the nature of the interference fringes produced has been elucidated. This achievement has led to the idea of developing a method of holographic interference capable of measuring lateral displacement in a two-dimensional plane and applying this method to a straightness meter. In this case, in order for the hologram to serve effectively as a straightness meter, realization of the real-time interference is an indispensable requirement. So far, the holographic interference method has been chiefly studied on the basis of the double-exposure technique because experiments by the real-time interference technique have been found very difficult. Besides, the conventional straightness meters utilizing the holographic interference method have unsolved problems regarding the instrumental accuracy, the effects of air disturbance and external vibrations, and the measurable range of distance to be traveled by their movable components along the optical axis. For the purpose of solving some of these problems, one of the inventors of this invention developed a new straightness meter utilizing holography which made it possible to realize real-time interference, alleviate the effects of air disturbance and external vibration, and to provide effective measurement of lateral displacement.
This previously developed holographic straightness meter is broadly composed of an optical unit to be mounted on a moving member such as a tool slide of the machine tool under measurement and an optical unit to be mounted on a fixed member opposed to the aforementioned moving member. The optical unit on the aforementioned fixed member comprises a laser generator, a beam splitter for dividing the beam from the laser into two beams, and a diffusing plate placed in the path of one of the divided beams. The optical unit on the moving member comprises a hologram and a screen disposed behind the hologram. By this straightness meter, the displacements of the moving member, i.e., the displacements in the horizontal direction and those in the vertical direction, in a plane perpendicular to the optical axis of the light of the object under test can be detected. In this holographic straightness meter, however, the length of the optical path from the diffusing plate to the hologram varies with the movement of the moving member and this variation affects the formation of interference fringes. For ideal operation of this straightness meter, therefore, it is necessary that the effects brought about on the formation of interference fringes by the movement of the movable member along the straight course of its movement should be eliminated. The accuracy of measurement by this meter is enhanced in proportion as the change in the number of interference fringes for fixed lateral displacement is increased. Thus, the straightness meter is desired to be so constructed that there appears a large change in the number of interference fringes to be observed.